1. Field of the Invention
The present invention relates to a fuel cell, comprising a membrane electrode assembly having a solid polymer electrolyte membrane, an anode side diffusion electrode, and a cathode side diffusion electrode, and a pair of separators holding the membrane electrode assembly, and in particular, to a fuel cell which can reliably seals the membrane electrode assembly between the separators, and which prevents a reaction gas from flowing around the membrane electrode assembly.
2. Description of the Related Art
In conventional fuel cells, the membrane electrode assembly comprises a solid polymer electrolyte membrane, and an anode side diffusion electrode and a cathode side diffusion electrode which are located at both sides of the membrane. The membrane electrode assembly is held by a pair of separators. By supplying fuel gas (for example, hydrogen gas) onto a reaction face of the anode side diffusion electrode, the hydrogen gas becomes ionized, and moves toward the cathode side diffusion electrode through the solid polymer electrolyte membrane. The electrons produced in this process are extracted to an external circuit, and are utilized as electric energy of a direct current. Since oxidant gas (for example, air which contains oxygen) is supplied to the cathode electrode, water is generated by the reaction of the hydrogen ions, the electrons, and the oxygen.
An example is explained with reference to FIG. 11. In FIG. 11, reference numeral 1 denotes the solid polymer electrolyte membrane. A fuel cell 4 is assembled such that the solid polymer electrolyte membrane 1 is held between gas diffusion electrodes (an anode side diffusion electrode and a cathode side diffusion electrode) 2 and 3. Sheet-type gaskets 5 which have openings corresponding to the reaction faces of the fuel cell 4 are provided at both sides of the fuel cell 4. While the gaskets 5 cover the edges of the fuel cell 4 and press the edges of the fuel cell 4 using outer pressers 6, the fuel cell 4 is held between separators 7 (disclosed in Japanese Unexamined Patent Application. First Publication No. Hei 6-325777).
As shown in FIG. 12, the rectangular fuel cell 4 is assembled such that the solid polymer electrolyte membrane 1 is held between the gas diffusion electrodes 2 and 3. A grooves 8 are formed in a pair of separators 7, and O-rings 9 are installed therein. The O-rings 9 supports the solid polymer electrolyte membrane 1 while the fuel cell 4 is held between the separators 7 and 7 (disclosed in Japanese Unexamined Patent Application. First Publication No. Hei 8-148169).
In the above conventional fuel cell, the gaskets 5 separate the spaces between the separators 7 and the gas diffusion electrodes 2 and 3 from the outside. Therefore, this fuel cell advantageously prevents the leakage of the fuel gas and the oxidant gas, and prevents the mixing of those gases, to thereby achieve efficient electric power generation. Variations in the thickness of the separators 7 and 8 and the gas diffusion electrodes 2 and 3 are unavoidable. Therefore, when the gaskets 5 which have varying thicknesses are combined with the separators 7 and the gas diffusion electrodes 2 and 3, the reaction force produced by the gaskets is not uniform. Thus, the problem is pointed out that the sealing between the separators 7 and the gas diffusion electrodes 2 and 3 is not uniform.
To achieve the reliable sealing, the manufacturing sizes of the separators 7 and the gas diffusion electrodes 2 and 3 must be accurately controlled, and, as a result, manufacturing costs are increased.
The above-mentioned fuel cell has problems in that the surface pressure of the gasket 5 varies in the peripheries of the openings of the separators 7, and in that a partial bending stress acts in the peripheries of the openings.
Particularly, when the fuel cell is provided in a vehicle, the thickness of the separator 7 must be adjusted so as to set the bending stress acting on the separator 7 with respect to the varying surface pressure of the gasket 5 less than a predetermined value. In this case, the fuel cell stack in which a number of fuel cells are stacked is large, and reduces the cabin.
Further, in the second conventional fuel cell, the O-rings securely seal the space C between the separators 7 and the fuel cell 4. However, the gas leaking from the end faces of the gas diffusion electrodes 2 and 3 does not contribute to the reaction, and flows onto the end surfaces of the gas diffusion electrodes 2 and 3, thus reducing the efficiency of the electric power generation.